Add more tests

This module probably needs more tests. Not sure exactly what yet, but we can track it here on this issue.

Here is some sample node body text I have been using to test whether the filters are working, by the way. I have a filter set up with:
custom: "Level" = "Beginner", "Intermediate", "Advanced" (with lower-case machine names)
module: bogus-7.x-3.3

And I use this text to test whether everything is working (of course, it isn't checking all the operators, but at least it checks basic functionality) -- could be used as a starting point for tests, if you don't already have one:

This is a test page.

[condition installed views > 7.x-3.0]
<p>This part should only display if views is installed, which it isn't.</p>
[/condition]
[condition installed bogus > 7.x-3.0]
<p>This part should only display if the bogus module is said to be installed and after 7.x-3.0, which it is.</p>
[/condition]
[condition installed bogus]
<p>This part should only display if the bogus module is said to be installed, which it is.</p>
[/condition]
[condition installed notdefined]
<p>This part should only display if the notdefined module is said to be installed, which it is not.</p>
[/condition]
[condition installed block]
<p>This part should display if Block is enabled, which it is.</p>
[/condition]
[condition installed block >= 7.0]
<p>This part should display if Block is enabled and >= 7.0, which it is</p>
[/condition]
[condition installed block == 7.2]
<p>This part should display if Block is enabled and version 7.2 exactly.</p>
[/condition]
[condition custom level beginner]
<p>This is beginner-level text</p>
[/condition]
[condition custom level intermediate]
<p>This is intermediate-level text</p>
[/condition]
[condition custom level advanced]
<p>This is advanced-level text</p>
[/condition]